Choose from any parts that lower your truck and see the APPROXIMATE look it will give you. Shown with stock wheels and tires
NOTE: At some point your rear axle will hit the frame if you are lowering it a lot, and a c-notch will be needed.

For the front: use the distance from the ground to the top of the frame rail as your guide, NOT the distance from your trucks current height.
For the rear: use the distance from the ground to the top of the bed wood.

Your trucks springs might already be sagging from age and use: Example: If your truck is already sagging 2", then a 3" drop spring will only lower it 1 more inch. Lowered measurments are from stock, original height